3.6 Redirections

Before a command is executed, its input and output
may be redirected
using a special notation interpreted by the shell.
Redirection allows commands’ file handles to be
duplicated, opened, closed,
made to refer to different files,
and can change the files the command reads from and writes to.
Redirection may also be used to modify file handles in the
current shell execution environment. The following redirection
operators may precede or appear anywhere within a
simple command or may follow a command.
Redirections are processed in the order they appear, from
left to right.

Each redirection that may be preceded by a file descriptor number
may instead be preceded by a word of the form {varname}.
In this case, for each redirection operator except
>&- and <&-, the shell will allocate a file descriptor greater
than 10 and assign it to {varname}. If >&- or <&- is preceded
by {varname}, the value of varname defines the file
descriptor to close.

In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
‘<’, the redirection refers to the standard input (file
descriptor 0). If the first character of the redirection operator
is ‘>’, the redirection refers to the standard output (file
descriptor 1).

The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace expansion,
tilde expansion, parameter expansion, command substitution, arithmetic
expansion, quote removal, filename expansion, and word splitting.
If it expands to more than one word, Bash reports an error.

Note that the order of redirections is significant. For example,
the command

ls > dirlist 2>&1

directs both standard output (file descriptor 1) and standard error
(file descriptor 2) to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist,
because the standard error was made a copy of the standard output
before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in
redirections, as described in the following table:

/dev/fd/fd

If fd is a valid integer, file descriptor fd is duplicated.

/dev/stdin

File descriptor 0 is duplicated.

/dev/stdout

File descriptor 1 is duplicated.

/dev/stderr

File descriptor 2 is duplicated.

/dev/tcp/host/port

If host is a valid hostname or Internet address, and port
is an integer port number or service name, Bash attempts to open
the corresponding TCP socket.

/dev/udp/host/port

If host is a valid hostname or Internet address, and port
is an integer port number or service name, Bash attempts to open
the corresponding UDP socket.

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used with
care, as they may conflict with file descriptors the shell uses
internally.

3.6.1 Redirecting Input

Redirection of input causes the file whose name results from
the expansion of word
to be opened for reading on file descriptor n,
or the standard input (file descriptor 0) if n
is not specified.

The general format for redirecting input is:

[n]<word

3.6.2 Redirecting Output

Redirection of output causes the file whose name results from
the expansion of word
to be opened for writing on file descriptor n,
or the standard output (file descriptor 1) if n
is not specified. If the file does not exist it is created;
if it does exist it is truncated to zero size.

The general format for redirecting output is:

[n]>[|]word

If the redirection operator is ‘>’, and the noclobber
option to the set builtin has been enabled, the redirection
will fail if the file whose name results from the expansion of
word exists and is a regular file.
If the redirection operator is ‘>|’, or the redirection operator is
‘>’ and the noclobber option is not enabled, the redirection
is attempted even if the file named by word exists.

3.6.3 Appending Redirected Output

Redirection of output in this fashion
causes the file whose name results from
the expansion of word
to be opened for appending on file descriptor n,
or the standard output (file descriptor 1) if n
is not specified. If the file does not exist it is created.

The general format for appending output is:

[n]>>word

3.6.4 Redirecting Standard Output and Standard Error

This construct allows both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to be redirected to the file whose name is the
expansion of word.

There are two formats for redirecting standard output and
standard error:

&>word

and

>&word

Of the two forms, the first is preferred.
This is semantically equivalent to

>word 2>&1

When using the second form, word may not expand to a number or
‘-’. If it does, other redirection operators apply
(see Duplicating File Descriptors below) for compatibility reasons.

3.6.5 Appending Standard Output and Standard Error

This construct allows both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to be appended to the file whose name is the
expansion of word.

The format for appending standard output and standard error is:

&>>word

This is semantically equivalent to

>>word 2>&1

(see Duplicating File Descriptors below).

3.6.6 Here Documents

This type of redirection instructs the shell to read input from the
current source until a line containing only word
(with no trailing blanks) is seen. All of
the lines read up to that point are then used as the standard
input for a command.

The format of here-documents is:

<<[-]wordhere-documentdelimiter

No parameter and variable expansion, command substitution,
arithmetic expansion, or filename expansion is performed on
word. If any characters in word are quoted, the
delimiter is the result of quote removal on word,
and the lines in the here-document are not expanded.
If word is unquoted,
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence \newline is ignored, and ‘\’
must be used to quote the characters
‘\’, ‘$’, and ‘`’.

If the redirection operator is ‘<<-’,
then all leading tab characters are stripped from input lines and the
line containing delimiter.
This allows here-documents within shell scripts to be indented in a
natural fashion.

3.6.7 Here Strings

A variant of here documents, the format is:

<<< word

The word undergoes
brace expansion, tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal.
Pathname expansion and word splitting are not performed.
The result is supplied as a single string to the command on its
standard input.

3.6.8 Duplicating File Descriptors

The redirection operator

[n]<&word

is used to duplicate input file descriptors.
If word
expands to one or more digits, the file descriptor denoted by n
is made to be a copy of that file descriptor.
If the digits in word do not specify a file descriptor open for
input, a redirection error occurs.
If word
evaluates to ‘-’, file descriptor n is closed.
If n is not specified, the standard input (file descriptor 0) is used.

The operator

[n]>&word

is used similarly to duplicate output file descriptors. If
n is not specified, the standard output (file descriptor 1) is used.
If the digits in word do not specify a file descriptor open for
output, a redirection error occurs.
If word
evaluates to ‘-’, file descriptor n is closed.
As a special case, if n is omitted, and word does not
expand to one or more digits or ‘-’, the standard output and standard
error are redirected as described previously.

3.6.9 Moving File Descriptors

The redirection operator

[n]<&digit-

moves the file descriptor digit to file descriptor n,
or the standard input (file descriptor 0) if n is not specified.
digit is closed after being duplicated to n.

Similarly, the redirection operator

[n]>&digit-

moves the file descriptor digit to file descriptor n,
or the standard output (file descriptor 1) if n is not specified.

3.6.10 Opening File Descriptors for Reading and Writing

The redirection operator

[n]<>word

causes the file whose name is the expansion of word
to be opened for both reading and writing on file descriptor
n, or on file descriptor 0 if n
is not specified. If the file does not exist, it is created.